In the telecommunications field, it is often desirable and frequently necessary to ascertain certain transmission characteristics, such as relative group delay, linearity, differential phase and gain, of a microwave radio communication link (MRCL). In particular, the slope of the system linearity affects the transmission characteristic. The practical methods which are most commonly employed today to make field measurements of radio transmission characteristics use commercially available transmission test sets. Typical of such commercial equipment are GTE Italia CSM 221/C - 222/C Radio Link Test Set (Milan, Italy), the Hewlett-Packard 3710/3702 Link Analyzer (Palo Alto, Calif.), and the Siemens K1005/K1046 Sweep Frequency Test Sets. While such test sets can be used to perform the required testing of performance, the disadvantages are the requirement for additional, expensive equipment and the problem of having the test equipment at the transmitter and receiver ends of the system. These disadvantages are overcome by having the facility built-in and at relatively nominal cost.